A conventional video production switcher receives several input video signals and combines them under control of one or more control signals to generate an output video signal. For example, the switcher might receive two full-field program video signals and two full-field matte signals and the control functions might enable the output video signal to be formed by placing a portion of the scene represented by video 1 into a selected area of the scene represented by video 2, or replacing the background of the video 1 scene with matte 1 or matte 2. In the conventional production switcher, each mixing function is accomplished using a video mixer which receives two video signals (either or both of which might be matte signal) and a control signal. If the video signals received by the mixer are video 1 and matte 1 and the control signal is control 1, and the dynamic range of each signal is limited to 1 volt within the active line interval, then the voltage of the mixer's output video signal during the active line interval is EQU video 1.times.control 1+matte 1 (1-control 1)
For the sake of simplicity, a reference hereinafter to a video or matte signal may include not only the signal itself but also the scene represented by the signal.
Because a wide range of effects can be carried out using a production switcher, the waveforms of the control signals that are applied to the mixers of a production switcher are potentially very complex.
One of the effects that can be accomplished using a production switcher is a vertical wipe. When an input signal representing a first scene is wiped vertically to an input signal representing a second scene, the scene represented by the output signal changes from the first scene to the second scene over several frames, and the transition between scenes is horizontal. A vertical wipe is accomplished using a control signal of which the voltage level remains essentially constant during any one line interval, the voltage level being 1 volt at one edge of the frame, e.g. the upper edge, being 0 volts at the opposite (lower) edge of the frame, and having a linear transition between 1 volt and 0 volts over a few lines at a vertical level that progresses from top to bottom of the screen. In the region within which the control signal changes from 1 volt to 0 volts, the two video signals are mixed in linear fashion.
The vertical wipe is a relatively simple effect, but carrying out a vertical wipe using a conventional production switcher uses substantialy mixing resources of the switcher, and therefore precludes use of the switcher to carry out more complex and demanding effects.